People gather outside Manchester Arena and station in Manchester, England, on Monday night. Peter Bendicho

by ANDREW BLANKSTEIN, ROBERT WINDREM, ALEX JOHNSON and TOM WINTER

At least 19 people were killed and about 50 others were injured in what British Prime Minister Theresa May called "an appalling suicide attack" after an Ariana Grande concert Monday night at Manchester Arena in England, authorities said.

Greater Manchester Police said the incident was being treated as terrorism. A senior White House official told NBC News that President Donald Trump, who is on a visit to Jerusalem, had been briefed.

Police said the explosion took place outside the arena, which is near the Manchester Victoria transit station, as the concert ended at about 10:35 p.m. local time (5:35 p.m. ET), catching people as they exited.

Multiple senior U.S. law enforcement officials briefed by British authorities told NBC News that forensic evidence at the scene — including a body found at the blast site — indicated a suicide attack. U.S. officials said initial reports indicated that some of the casualties might have been caused by a stampede of concert-goers.

"My thoughts are with the victims and the families of those affected in what is being treated by the police as an appalling terrorist attack," May said in a brief statement.

A representative of Ariana Grande said the singer, who has millions of young fans around the world, was not hurt. Likewise, Bianca Landrau, the Boston hiphop star known as Bia, who also performed, tweeted: "Guys we are okay!!!"

Billy Graham would be the first to say that his message was not complex or unique, but he won over audiences worldwide with his friendliness, openness, humility and unyielding religious conviction.More

Billy Graham would be the first to say that his message was not complex or unique, but he won over audiences worldwide with his friendliness, openness, humility and unyielding religious conviction.More